From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Date: Fri, 11 Feb 2022 09:41:30 -0800 > On Wed, Feb 09, 2022 at 07:57:39PM +0100, Alexander Lobakin wrote: > > Position-based search, which means that if there are several symbols > > with the same name, the user needs to additionally provide the > > "index" of a desired symbol, is fragile. For example, it breaks > > when two symbols with the same name are located in different > > sections. > > > > Since a while, LD has a flag `-z unique-symbol` which appends > > numeric suffixes to the functions with the same name (in symtab > > and strtab). It can be used to effectively prevent from having > > any ambiguity when referring to a symbol by its name. > > In the patch description can you also give the version of binutils (and > possibly other linkers) which have the flag? Yeah, sure. > > > Check for its availability and always prefer when the livepatching > > is on. It can be used unconditionally later on after broader testing > > on a wide variety of machines, but for now let's stick to the actual > > CONFIG_LIVEPATCH=y case, which is true for most of distro configs > > anyways. > > Has anybody objected to just enabling it for *all* configs, not just for > livepatch? A few folks previously. > > I'd much prefer that: the less "special" livepatch is (and the distros > which enable it), the better. And I think having unique symbols would > benefit some other components. Agree, I just want this series to be as least invasive for non-FG-KASLR builds as possible. And currently this flag make depmod emit a bunch of harmless false-positive warnings, so I'd wait until at least the series is accepted / I post a patch for depmod and it gets accepted. > > > +++ b/kernel/livepatch/core.c > > @@ -143,11 +143,13 @@ static int klp_find_callback(void *data, const char *name, > > args->count++; > > > > /* > > - * Finish the search when the symbol is found for the desired position > > - * or the position is not defined for a non-unique symbol. > > + * Finish the search when unique symbol names are enabled > > + * or the symbol is found for the desired position or the > > + * position is not defined for a non-unique symbol. > > */ > > - if ((args->pos && (args->count == args->pos)) || > > - (!args->pos && (args->count > 1))) > > + if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL) || > > + (args->pos && args->count == args->pos) || > > + (!args->pos && args->count > 1)) > > return 1; > > There's no real need to do this. The code already works as-is, even if > there are no unique symbols. > > Even if there are no duplicates, there's little harm in going through > all the symbols anyway, to check for errors just in case something > unexpected happened with the linking (unexpected duplicate) or the patch > creation (unexpected sympos). It's not a hot path, so performance isn't > really a concern. > > When the old linker versions eventually age out, we can then go strip > out all the sympos stuff. > > > @@ -169,6 +171,13 @@ static int klp_find_object_symbol(const char *objname, const char *name, > > else > > kallsyms_on_each_symbol(klp_find_callback, &args); > > > > + /* > > + * If the LD's `-z unique-symbol` flag is available and enabled, > > + * sympos checks are not relevant. > > + */ > > + if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL)) > > + sympos = 0; > > + > > Similarly, I don't see a need for this. If the patch is legit then > sympos should already be zero. If not, an error gets reported and the > patch fails to load. Right, but for both those chunks the main idea is to let the compiler optimize-out the code non-actual for unique-symbol builds: add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-80 (-77) Function old new delta klp_find_callback 139 142 +3 klp_find_object_symbol.cold 85 48 -37 klp_find_object_symbol 168 125 -43 > > -- > Josh Thanks, Al
